Is there an existing Shopping Cart

I'm afraid you will have to do it on your own. Mostly shopping carts have to
much details from shop to shop that makes them drastically different and
it's something I would allways do from scratch because of those reasons.
Shopping cart on it's simplest version is few hours of work and IMHO doesn't
work for anyone so that's why there are no such components…

When you say shopping cart I'm thinking only on shopping cart not checkout
which can include shipping methods/payment methods/giftwrapping coupons and much
more, but that's not shopping cart.